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Comment by boloust | Hacker News Reader

> Frankly, if you were to ask me what the top 3 sources of my headaches in Python are, number 1, 2, and 3 would be, receiving a None value when I expected something else

Curious example, because that's absolutely something a sufficiently advanced and well-thought-out static type system can completely solve.

You're right that the type systems of C and Java do not meet this bar, however.

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Is null even a type though? If not, then it would seem it's not a static type system that's solving it; it's something else entirely solving it (whether or not that something is contained in that specific type system).

The issue with null, as commonly implemented, is that null is a value that inhabits all types. So in that sense it is a defect of the type system.

In Common Lisp, T is the "any" type (which makes sense there, as type specifiers can be formed using unions using (or type-A type-B ...) lists), the value NIL is the sole instance of the type NULL.

Receiving a None or Nothing or a NULL or similar is something a type system can paper over or, more politely, force you to add code to ward off but there's something happening in the actual data, if not the real world, which is causing that non-value to be present and the type system can't fix that.

But is the issue receiving a null or is it how unexpected the null is? Assuming that functions always can return null is way too tedious, so in practice my issue with dynamic languages like Python is that I'm never sure whether a function can actually return null.

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